Intercalation-induced partial exfoliation of NiFe LDHs with abundant active edge sites for highly enhanced oxygen evolution reaction

Synthesis of Zr2ON2 via a urea-glass route to modulate the bifunctional catalytic activity of NiFe layered double hydroxide in a rechargeable zinc-air battery

The development of a highly efficient, stable, and low-cost bifunctional catalyst is imperative for facilitating the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, significant challenges are involved in extending its applications to rechargeable zinc-air batteries. This study presents a bifunctional catalyst, Zr2ON2@NiFe layered double hydroxide (LDH), that was developed by utilizing a urea-glass route for synthesizing the Zr2ON2 precursor, followed by riveting NiFe LDH nanosheets using a hydrothermal method. Specifically, the vertical distribution of NiFe LDH on the Zr2ON2 surface ensures the maximization of the number of accessible active sites and interfacial catalysis of NiFe LDH. Notably, Zr2ON2@NiFe LDH demonstrates ORR and OER bifunctional electrocatalytic behavior and high stability owing to its heterostructure and composition. Furthermore, a rechargeable zinc-air battery using a Zr2ON2@NiFe LDH electrocatalyst as the air cathode demonstrated a high peak power density (172 mW cm-2) and galvanostatic charge-discharge cycle stability (5 mA cm-2 over 443 h). Thus, this study presents an efficient and cost-effective strategy for the design of bifunctional electrocatalysts.

Controllable Phase Separation Engineering of Iron-Cobalt Alloy Heterojunction for Efficient Water Oxidation

The tailor-made transition metal alloy-based heterojunctions hold a promising prospect for the electrocatalytic oxygen evolution reaction (OER). Herein, a series of iron-cobalt bimetallic alloy heterojunctions are purposely designed and constructed via a newly developed controllable phase separation engineering strategy. The results show that the phase separation process and alloy component distribution rely on the metal molar ratio (Fe/Co), indicative of the metal content dependent behavior. Theoretical calculations demonstrate that the electronic structure and charge distribution of iron-cobalt bimetallic alloy can be modulated and optimized, thus leading to the formation of an electron-rich interface layer, which likely tunes the d-band center and reduces the adsorption energy barrier toward electrocatalytic intermediates. As a result, the Fe0.25Co0.75/Co heterojunction exhibits superior OER activity with a low overpotential of 185 mV at 10 mA cm-2. Moreover, it can reach industrial-level current densities and excellent durability in high-temperature and high-concentration electrolyte (30 wt % KOH), exhibiting enormous potential for industrial applications.

Highly Stable Mo-NiO@NiFe-Layered Double Hydroxide Heterojunction Anode Catalyst for Alkaline Electrolyzers with Porous Membrane

Heterostructure catalysts are considered as promising candidates for promoting the oxygen evolution reaction (OER) process due to their strong electron coupling. However, the inevitable dissolution and detachment of the heterostructure catalysts are caused by the severe reconstruction, dramatically limiting their industrial application. Herein, the NiFe-layered double hydroxide (LDH) nanosheets attached on Mo-NiO microrods (Mo-NiO@NiFe LDH) by the preoxidation strategy of the core NiMoN layer are synthesized for ensuring the high catalytic performance and stability. Owing to the enhanced electron coupling and preoxidation process, the obtained Mo-NiO@NiFe LDH exhibits a superlow overpotential of 253 mV to achieve a practically relevant current density of 1000 mA cm-2 for OER with exceptional stability over 1200 h. Notably, the overall water splitting system based on Mo-NiO@NiFe LDH reveals remarkable stability, maintaining the catalytic activity at a current density of 1000 mA cm-2 for 140 h under industrial harsh conditions. Furthermore, the Mo-NiO@NiFe LDH demonstrates outstanding activity and long-term durability in a practical alkaline electrolyzer assembly with a porous membrane, even surpassing the performance of IrO2. This work provides a new sight for designing and synthesizing highly stable heterojunction electrocatalysts, further promoting and realizing the industrial electrocatalytic OER.

In-situ fabrication of Cr doped FeNi LDH on commercial stainless steel for oxygen evolution reaction

Commercial stainless steel has attracted increasing interest due to their rich content in transition metal elements and corrosion resistance properties. In this work, we design a facile and rapid route to in-situ fabricate the Cr doped FeNi layered double hydroxides nanosheets (LDHs) on modified stainless steel (Cr-FeNi LDH @ ESS) under ambient condition.The ultra small scaled 2D structure only around 20 nm diameter and metal ions with multivalent oxidation state were observed on the in situ fabricated LDHs, which provides high active area and active sites and thus promote excellent oxygen evolution reaction (OER). The Cr-FeNi LDH @ESS electrocatalysts exhibit an over potential of 280 mV at 10 mA cm-2 and achieves a Tafel slope of 44 mV dec-1 for OER in the 1.0 M KOH aqueous solution. We anticipate that the operating strategy of our system may promote the development of commercial non-precious productions as the efficient electrocatalysts for energy storage and conversion.

Interface strong-coupled 3D Mo-NiS@Ni-Fe LDH flower-cluster as exceptionally efficient electrocatalyst for water splitting in wide pH range

It is crucial to create a bifunctional catalyst with high efficiency and low cost for electrochemical water splitting under alkaline and neutral pH conditions. This study investigated the in-situ creation of ultrafine Mo-NiS and NiFe LDH nanosheets as an effective and stable electrocatalyst with a three-dimensional (3D) flower-cluster hierarchical structure (Mo-NiS@NiFe LDH). The strong interfacial connection between Mo-NiS and NiFe LDH enhances the formation of metal higher chemical states in the material, optimizes the electronic structure, increases OH^- adsorption capacity improves electron transfer/mass diffusion, and promotes O₂/H₂ gas release. As a result, at 10 mA cm^-2, Mo-NiS@NiFe LDH/NF demonstrates the outstanding bifunctional electrocatalytic activity of just 107 mV (HER, hydrogen evolution reaction) and 184 mV (hydrogen evolution reaction) (OER, oxygen evolution reaction). The catalytic performance is remarkably stable after 72 h of continuous operation in 1 M KOH at high current densities (300 mA cm^-2). More interestingly, in the overall water splitting system, the cell voltages for anode and cathode in both alkaline and neutral electrolytes for Mo-NiS@NiFe LDH/NF are only 1.54 V (alkaline) and 2.06 V (neutral) at 10 mA cm^-2. These results demonstrated that the bifunctional electrocatalyst design concept is a viable solution for water splitting in both alkaline and neutral systems.

Recent Advances of Modified Ni (Co, Fe)-Based LDH 2D Materials for Water Splitting

Water splitting technology is an efficient approach to produce hydrogen (H₂) as an energy carrier, which can address the problems of environmental deterioration and energy shortage well, as well as establishment of a clean and sustainable hydrogen economy powered by renewable energy sources due to the green reaction of H₂ with O₂. The efficiency of H₂ production by water splitting technology is intimately related with the reactions on the electrode. Nowadays, the efficient electrocatalysts in water splitting reactions are the precious metal-based materials, i.e., Pt/C, RuO₂, and IrO₂. Ni (Co, Fe)-based layered double hydroxides (LDH) two-dimensional (2D) materials are the typical non-precious metal-based materials in water splitting with their advantages including low cost, excellent electrocatalytic performance, and simple preparation methods. They exhibit great potential for the substitution of precious metal-based materials. This review summarizes the recent progress of Ni (Co, Fe)-based LDH 2D materials for water splitting, and mainly focuses on discussing and analyzing the different strategies for modifying LDH materials towards high electrocatalytic performance. We also discuss recent achievements, including their electronic structure, electrocatalytic performance, catalytic center, preparation process, and catalytic mechanism. Furthermore, the characterization progress in revealing the electronic structure and catalytic mechanism of LDH is highlighted in this review. Finally, we put forward some future perspectives relating to design and explore advanced LDH catalysts in water splitting.

Triethanolamine-assisted synthesis of NiFe layered double hydroxide ultrathin nanosheets for efficient oxygen evolution reaction

Water electrolysis is a promising technique for producing high-quality hydrogen, the application of which is impeded by the sluggish oxygen evolution reaction (OER) process. In this study, ultrathin nickel-iron layered double hydroxide (NiFe LDH) nanosheets were successfully synthesized through a facile hydrothermal reaction with the assistance of triethanolamine (TEA). Morphological and structural characterizations revealed that the presence of TEA modified the morphology of NiFe LDH, facilitated the synthesis of high-purity NiFe LDH, improved the crystallinity of NiFe LDH and resulted in a slight decrease in specific surface area. X-ray photoelectron spectroscopy (XPS) analysis demonstrated the modulation of the electronic structure engendered by the addition of TEA, with nickel and iron appearing in high valence state in the resulting NiFe LDH nanosheets. The as-prepared NiFe LDH nanosheets possessed outstanding OER activity with fast kinetics, exhibiting a low overpotential of 261 mV to achieve a current density of 10 mA cm-2 and a small Tafel slope of 32.5 mV dec-1 in 1 M KOH. The excellent OER performance and rapid OER kinetics are mainly attributed to the high-valence Ni and Fe rather than the modification in the morphology and microstructure.

Effect of Surface‐Adsorbed and Intercalated (Oxy)anions on the Oxygen Evolution Reaction

As the kinetically demanding oxygen evolution reaction (OER) is crucial for the decarbonization of our society, a wide range of (pre)catalysts with various non‐active‐site elements (e.g., Mo, S, Se, N, P, C, Si…) have been investigated. Thermodynamics dictate that these elements oxidize during industrial operation. The formed oxyanions are water soluble and thus predominantly leach in a reconstruction process. Nevertheless, recently, it was unveiled that these thermodynamically stable (oxy)anions can adsorb on the surface or intercalate in the interlayer space of the active catalyst. There, they tune the electronic properties of the active sites and can interact with the reaction intermediates, changing the OER kinetics and potentially breaking the persisting OER *OH/*OOH scaling relations. Thus, the addition of (oxy)anions to the electrolyte opens a new design dimension for OER catalysis and the herein discussed observations deepen the understanding of the role of anions in the OER.

Designing hierarchical iron doped nickel-vanadium hydroxide microsphere as an efficient electrocatalyst for oxygen evolution reaction

Exploring highly active and inexpensive electrocatalysts for oxygen evolution reaction (OER) is considered to be one of the preconditions for the development of energy and environment-related technologies. Nickel-based layered double hydroxides (LDHs) are extensively-studied OER electrocatalysts, but they still require relatively high overpotentials to achieve threshold current densities. In this work, iron-doped nickel-vanadium hydroxide microspheres (Fe-doped NiV HMS) were synthesized by doping iron ions into the NiV HMS through a facile cation-exchange method. The Fe-doped NiV HMS are hollow hierarchical structure stacked by high-density perpendicularly-lying nanosheets, which provide enough space for electrolyte penetration and diffusion. Owing to optimized composition and hollow hierarchical structure, the Fe-doped NiV HMS exhibits excellent electrocatalytic performance, which possessed a very low running overpotential (255 mV at 10 mA cm^-2) and a smallest Tafel slope (56 mV dec^-1) compared with hierarchical NiV HMS toward OER. Electrochemical results and density functional theory (DFT) manifest that Fe doping could regulate the electronic structure of NiV HMS, thus improving its electrical conductivity and electron transfer rate, and thus enhancing its catalytic activity. This research provides a convenient way to prepare Ni-based hydroxides as promising OER catalysts.

Fundamental understanding of electrocatalysis over layered double hydroxides from the aspects of crystal and electronic structures

Layered double hydroxides (LDHs) composed of octahedral ligand units centered with various transition metal atoms display unique electronic structures and thus attract significant attention in the field of electrocatalytic oxygen evolution reactions (OER). Intensive experimental explorations have therefore been carried out to investigate the LDHs synthesis, amorphous control, intrinsic material modifications, interfacing with other phases, strain, etc. There is still the need for a fundamental understanding of the structure-property relations, which could hinder the design of the next generation of the LDHs catalysts. In this review, we firstly provide the crystal structure information accompanied by the corresponding electronic structures. Then, we discuss the conflicts of the active sites on the NiFe LDHs and propose the synergistic cooperation among the ligand units during OER to deliver a different angle for understanding the current structure-property relations beyond the single-site-based catalysis process. In the next section of the OER process, the linear relationship-induced theoretical limit of the overpotential is further discussed based on the fundamental aspects. To break up the linear relations, we have summarized the current strategies for optimizing the OER performance. Lastly, based on the understanding gained above, the perspective of the research challenges and opportunities are proposed.